qemu: command: Enforce setting XDG variables for system QEMU

For session mode, only XDG_CACHE_HOME is set, because we want to remain
integrating with services in user session, but for system mode, this
would have become reading/writing to '/' which carries the obvious issue
with permissions (also, '/' is the wrong location in 99.9% cases anyway).

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

tests/qemuxml2argvdata/graphics-spice-auto-socket-cfg.args

@@ -3,6 +3,9 @@ PATH=/bin \
 HOME=/home/test \
 USER=test \
 LOGNAME=test \
+XDG_DATA_HOME=/tmp/lib/domain--1-QEMUGuest1/.local/share \
+XDG_CACHE_HOME=/tmp/lib/domain--1-QEMUGuest1/.cache \
+XDG_CONFIG_HOME=/tmp/lib/domain--1-QEMUGuest1/.config \
 QEMU_AUDIO_DRV=spice \
 /usr/bin/qemu-system-i686 \
 -name QEMUGuest1 \